Polymers, such as polyolefins, polyesters, polycarbonates, polyamides, polyurethanes, epoxy resins, and other thermoplastic or thermoset polymer resins, are frequently made more flame retardant by incorporating therein a phosphorus-containing compound, a halogen-containing compound or a mixture thereof.
Some polymers are processed at high temperatures, for example 200° C., 220° C., 250° C. or higher, and many known flame retardants are not suitable under these conditions because they are too volatile, not sufficiently thermally stable, have an adverse effect on processing, etc. Certain organophosphorus flame retardant compounds, such as some phosphate esters, can also exhibit a plasticizing effect which may adversely affect mechanical properties of the polymers into which they are added. In addition, compounds such as some phosphates are relatively unstable to hydrolysis, which can result in undesired formation of various phosphoric acid compounds.
Salts of phosphorus containing acids are known flame-retardant additives, in particular for thermoplastic polymers. U.S. Pat. No. 3,894,986 discloses flame retardant thermoplastic polyesters containing alkali salts of phosphonic acids, e.g., the mono sodium salt of ethane-phosphonic acid or a sodium salt of a mono-methyl ester of an alkane-phosphonic acid. U.S. Pat. No. 4,972,011 discloses aluminum salts of alkylphosphonic acids or mono-alkyl esters of alkane-phosphonic acids, i.e., salts of compounds of formula (III), wherein R is for example methyl, ethyl, propyl or isopropyl etc., unsubstituted or substituted by one or more halo or hydroxy groups; and R′ is hydrogen, methyl, ethyl, propyl, or isopropyl
DE 3833977 discloses metal salts of compounds of formula (III) prepared from reactions of dimethyl methylphosphinate and metal oxides or hydroxides in water at high pressures and temperatures from 120 to 200° C.; reactions run in aqueous solution under elevated pressures at temperatures up to 190° C. in an autoclave are exemplified. Adducts of these salts with amines such as ethylene diamine and melamine, and use of the adducts as flame retardants in thermoplastics are also disclosed.
Salts of phosphinic acids, i.e., compounds of formula (II) wherein R1 and R2 are alkyl or carbon based aromatic, are also known flame-retardant additives for thermoplastic polymers.

Salts wherein M is selected from Mg, Ca, Al, Sb, Sn, Ge, Ti, Zn, Fe, Zr, Ce, Bi, Li, Na, K or protonated nitrogen base are known. For example, U.S. Pat. Nos. 5,780,534 and 6,013,707 disclose that calcium phosphinates and aluminum phosphinates of Formula (II) are particularly effective in polyester, for example, calcium and aluminum salts of dimethylphosphinic acid, ethylmethylphosphinic acid, diethylphosphinic acid, n-propylmethylphosphinic acid, n-propylethylphosphinic acid, di-n-propylphosphinic acid, diisopropylphosphinic acid or diphenylphosphinic acid.
U.S. Pat. No. 6,365,071 discloses a synergistic flame retardant combination for thermoplastic polymers, e.g., engineering plastics, especially for polyesters, comprising A) a phosphinic salt of the formula (II) above, e.g., aluminum dimethylphosphinate, aluminum methylethylphosphinate, and aluminum methylpropylphosphinate and B) a nitrogen compound such as allantoin, i.e., (2,5-dioxo-4-imidazolidinyl) urea, benzoguanamine, glycoluril, i.e., tetrahydroimidazo[4,5-d]imidazole-2,5-dione, urea cyanurate, melamine cyanurate and melamine phosphate. U.S. Pat. No. 6,255,371 discloses a flame retardant combination comprising, A) a phosphinate of formula (II) above, e.g., a diethyl phosphinate where M is calcium, magnesium, aluminum and/or zinc, and B) condensation or reaction products of melamine e.g., melamine polyphosphate, melam polyphosphate and melem polyphosphate.
U.S. Pat. No. 6,547,992 discloses a flame retardant combination for thermoplastic polymers comprising phosphinates and small amounts of inorganic and/or mineral compounds which do not contain nitrogen. WO 2012/045414 discloses a flame retardant composition comprising A) a phosphinic salt of the formula (II) above wherein M is selected from Mg, Ca, Al, Sb, Sn, Ge, Ti, Zn, Fe, Zr, Ce, Bi, Li, Na, K or a protonated nitrogen base; B) a metal salt of phosphonic acid; and other optional components.
The phosphinates cited above, e.g., U.S. Pat. Nos. 6,365,071 and 6,255,371, are said to be thermally stable, and to not adversely affect polymers during processing. The phosphinates are not volatile under the customary conditions of preparation and processing of thermoplastic polymers. However, these materials are not necessarily suitable for use in all polymer systems and may create problems for processing or may lack the flame retardant effectiveness needed for certain polymers. There is still a need for flame retardants with greater efficiency at lower additive concentrations and improved processability for use in preparing flame retardant polymer compositions with highly desirable physical properties.
Phosphonic acid salts, i.e., salts metal salts of compounds according to formula (Ia), are also reported to be thermally stable, but this is of course a relative term. As disclosed in US 2007/0029532, decomposition of such phosphonic acid salts is well known at temperatures encountered during processing of polyesters and polyamides, damaging the polymers in the process.
Copending U.S. patent application Ser. Nos. 14/337,500 and 14/592,472, disclose that heating alkylphosphonic acid metal salts the formula:
wherein R is alkyl, aryl and the like, e.g., aluminum salts, calcium salts, zinc salts etc., of methyl, ethyl, propyl, phenyl or benzyl phosphonic acid at temperatures in excess of 200° C. produces flame retardant materials that, unlike their precusors, are thermally stable at temperatures above 400° C. and can be thermally incorporated onto thermoplastic polymer resins without adversely impacting the physical properties of the polymer.
U.S. Pat. No. 4,879,327 discloses amine salts of phosphonic acid, i.e., s-triazine, 1,2,4-triazole, benzimidazole, heptazine, pyrimidine, or piperazine salts, which may be combined with a polyhydroxylated compound as fireproofing agents for plastics, e.g., polyamides and polyolefins.
Metal and ammonium salts of pyrophosphonic acid are known. U.S. Pat. No. 4,859,466 discloses that pyrophosphonic salts of formula (V) have microbiocide activity.

U.S. Pat. No. 4,267,098 discloses copolymers of ethylene and tetrafluoroethylene with improved heat stability comprising 0.001 to 2 wt % by weight of a metal salt of phosphonic acid, H2(HPO3), diphosphonic acid, H2(H2P2O5), hypodiphosphonic acid H2(H2P2O4), or hypophosphonic acid H(H2PO2), wherein the metal is selected from the first to third main group of the periodic table of the elements, e.g., zinc or manganese.